The benefits of sustained release pharmaceutical preparations are well known in the art. Many therapeutic agents have a short half-life and or are rapidly cleared (metabolised) from mammalian bodies requiring frequent or repeated administration of the therapeutic agent to bring about a therapeutic effect in a patient.
A wide variety of different sustained release pharmaceutical preparations are known in the art. Some sustained release pharmaceutical preparations are “matrix” type, and comprise an active compound dispersed in a matrix of a carrier material. The carrier material may be porous or non-porous, solid or semi-solid, and permeable or impermeable to the active compound. Matrix devices may be biodegradable, i.e., they may slowly erode after administration. Alternatively, matrix devices may be nondegradable, and rely on-diffusion of the active compound through the walls or pores of the matrix. Matrix devices may be easily prepared, but are not suitable for all compounds. Furthermore, it is difficult to prepare matrix devices that release active compound at a constant rate (i.e., zero order kinetics). Generally, the release rate is typically a function of the active compound's concentration in the matrix.
There are however, many problems with existing implants when used to administer therapeutic agents to patients. First, they deliver a high dose of active agent within a first few days of implantation before reaching a plateau rate of delivery for the majority of the lifespan of the implant. This is due to an initially high rate of absorption caused by the breakdown of the surface of the implant.
Second, irritation of tissues surrounding the implant site occurs because of the initial high rate of absorption of the active agent. This results in redness and soreness of the skin immediately around the implant.
Third, there is the problem that a constant rate of release of active agent over a long period of time is difficult to achieve. Generally, the release rate is typically a function of the concentration of the active agent.
A final problem is that existing implants have a relatively short lifespan.
Thus, there is a need for an improved implant, which deliver active agent at a therapeutic rate without an initial rise in concentration of the active agent for an extended period of time.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.